A multi-channel communication system utilizes multiple “transmission” channels for data transmission. These transmission channels may be formed in the time domain, frequency domain, spatial domain, or a combination thereof. For example, the multiple transmission channels may correspond to different time slots in a time division multiplex (TDM) communication system, different frequency subbands in an orthogonal frequency division multiplex (OFDM) communication system, or different spatial channels in a multiple-input multiple-output (MIMO) communication system. The TDM, OFDM and MIMO systems are described in further detail below.
The multiple transmission channels in the multi-channel communication system may experience different channel conditions (e.g., different fading, multipath and interference effects) and may achieve different signal-to-noise-and-interference ratios (SNRs). The SNR of a transmission channel determines its transmission capability, which is typically quantified by a particular transmission rate that may be reliably achieved on the transmission channel. If the SNR varies from transmission channel to transmission channel, then the supported transmission rate would also vary from channel to channel. Moreover, since the channel conditions typically vary with time, the transmission rates supported by the transmission channels would also vary with time.
A major challenge in a coded communication system is selecting the appropriate transmission mode(s) to use for data transmission based on the channel conditions. As used herein, a “transmission mode” may indicate a particular transmission rate or information bit rate, a particular coding scheme, a particular modulation scheme, or a combination thereof, to use for a given data transmission. The goal of the transmission mode selection should be to maximize throughput on the multiple transmission channels while meeting certain quality objectives, which may be quantified by a particular packet error rate (PER). Each transmission channel may also be referred to as a spatial channel. Each spatial channel may carry a data transmission, herein referred to as a spatial stream or spatial data stream.
One straightforward technique is to select a particular transmission mode for each of the multiple transmission channels based on its SNR (i.e., the transmission mode selection is done on a per transmission channel basis to “bit load” each transmission channel according to its SNR). The data for each transmission channel would then be sent at the transmission rate and with the coding and modulation schemes associated with the transmission mode selected for that transmission channel. However, this technique has some major drawbacks. First, coding individually for each transmission channel can significantly increase the complexity of the data processing at both a transmitter and a receiver. Second, coding individually for each transmission channel may greatly increase coding and decoding delay. Third, a high feedback rate may be needed to send back information (e.g., the SNR or transmission mode) for each transmission channel, which is needed by the transmitter to code and modulate data on a channel-by-channel basis.
Another technique is to use a common transmission mode for all spatial data streams or groups of spatial data streams. This technique avoids the major drawbacks of the bit-loading technique. However, if a data transmission is sent on multiple transmission channels with varying SNRs, then the SNR would vary correspondingly across the received data transmission. The challenge is then to select the proper transmission mode to use for the data transmission in light of the varying SNRs, so that the data transmission can be reliably received. If the transmission rate for the selected transmission mode is too high, then the entire data transmission would be received in error. Conversely, if the transmission rate for the selected transmission mode is too low, then the transmission capacity of the multiple transmission channels is under utilized.
There is therefore a need in the art for techniques to determine a suitable transmission mode for data transmission on multiple transmission channels having varying SNRs.